Related Articles
User Defined Literals in C++
• Difficulty Level : Hard
• Last Updated : 07 Sep, 2018

User Defined Literals (UDL) are added in C++ from C++11. Although, C++ provides literals for a variety of built-in types but these are limited.

Examples of literals for built-in types :

```// Examples of classical literals for built-in types.
42    // int
2.4    // double
3.2F    // float
'w'    // char
32ULL    // Unsigned long long
0xD0    // Hexadecimal unsigned
"cd"    // C-style string(const char")```

Why do we use UDLs?
Let us consider below example to understand need of UDLs.

```long double Weight = 2.3; //  pounds? kilograms? grams?

// With UDL, we attach units to the values which has
// following advantages
// 1) The code becomes readable.
// 2) Conversion computations are done at compile time.
weight = 2.3kg;
ratio = 2.3kg/1.2lb;```

To compute the above ratio it is necessary to convert them to same units. UDLs help us to overcome unit translation cost. We can define user-defined literals for user-defined types and new form of literals for built-in types. They help to make constants in code more readable. The value of UDLs is substituted with the actual value defined in the code by the compiler at compile time. UDL’s don’t save much of coding time but more and more calculations can be shifted to compile-time for faster execution.

Examples of User Defined Literals:

```"hello"s            // string
4.3i                // imaginary
101000111101001b    // binary
53h                // hours
234093270497230409328432840923849 // extended-precision```

UDLs are treated as a call to a literal operator. Only suffix form is supported. The name of the literal operator is operator “” followed by the suffix.

Example 1:

 `// C++ code to demonstrate working of user defined``// literals (UDLs)``#include``#include``using` `namespace` `std;`` ` `// user defined literals`` ` `// KiloGram``long` `double` `operator``""` `_kg( ``long` `double` `x )``{``    ``return` `x*1000;``}`` ` `// Gram``long` `double` `operator``""` `_g( ``long` `double` `x )``{``    ``return` `x;``}`` ` `// MiliGram``long` `double` `operator``""` `_mg( ``long` `double` `x )``{``    ``return` `x / 1000;``}`` ` `// Driver code``int` `main()``{``    ``long` `double` `weight = 3.6_kg;``    ``cout << weight << endl;``    ``cout << setprecision(8) << ( weight + 2.3_mg ) << endl;``    ``cout << ( 32.3_kg / 2.0_g ) << endl;``    ``cout << ( 32.3_mg *2.0_g ) << endl;``    ``return` `0;``}`
Output:
```3600
3600.0023
16150
0.0646
```

Example 2:

 `#include ``#include ``using` `namespace` `std;`` ` `// imaginary literal``constexpr complex <``double``> operator``""` `_i( ``long` `double` `d )``{``    ``return` `complex <``double``> { 0.0 , ``static_cast` `<``double``> ( d ) };``}`` ` `int` `main()``{``    ``complex <``double``> z = 3.0 + 4.0_i;``    ``complex <``double``> y = 2.3 + 5.0_i;``    ``cout << ``"z + y = "` `<< z+y << endl;``    ``cout << ``"z * y = "` `<< z*y << endl;``    ``cout << ``"abs(z) = "` `<< ``abs``(z) << endl;``    ``return` `0;``}`
Output:
```z + y = (5.3,9)
z * y = (-13.1,24.2)
abs(z) = 5
```

Here, constexpr is used to enable compile time evaluation.

Restriction:
UDL can only work with the following parameters:

``````char const*
unsigned long long
long double
char const*, std::size_t
wchar_t const*, std::size_t
char16_t const*, std::size_t
char32_t const*, std::size_t``````

But return value can be of any types.

This article is contributed by Mahima Varshney. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Want to learn from the best curated videos and practice problems, check out the C++ Foundation Course for Basic to Advanced C++ and C++ STL Course for foundation plus STL.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.
My Personal Notes arrow_drop_up